The present invention relates generally to aircraft landing gears, and more specifically to systems for prerotating the landing gear wheel prior to landing.
Many patents have been issued to date relating generally to aircraft landing wheel prerotation devices and schemes. Extending back as far as the 1920's, many of these patents should be viewed in light of the sizes and speeds of airplanes contemporary with those devices. With today's aircraft typically being vastly larger, heavier and faster, many of those devices of yesteryear would self-destruct if used in their patented form.
Notwithstanding this veritable avalanche of attention and patent coverage, it is this inventor's understanding that no commercial or other implementation of any such prerotation device has come about at this time, reflecting a long felt but unsolved need in this art.
Generally, past inventors' work, as evidenced by their patents, falls into three categories: 1) prerotation induced by affixing some manner of air-catching appendage to the aircraft wheel and/or modifying the wheel itself to act as an impeller; 2) prerotation requiring some manner of electric or mechanical power from the aircraft itself, transmitted to the landing wheels through some form of mechanism, power train or linkage; or 3) prerotation induced by modification of the aircraft tire by air-catching appendages bonded to the tire sidewalls or molded integrally with the tire. Some of the air-driven types included a baffle or a tube to cause more air to impinge upon these appendages.
One patent, however, Opitz U.S. Pat. No. 3,866,860, 1975, consisted of a sort of fender enclosing the upper half of the aircraft wheel so that only the lower half would be exposed to the airstream, thereby encouraging some rotation through the friction of the passing air. Others, including Pogue, U.S. Pat. No. 2,072,277, 1937, suggested a sort of caterpillar tractor-like appendage with a flexible belt traveling around several rollers. Livermon, U.S. Pat. No. 2,467,140, 1949, proposed to use explosive charges timed to explode into cavities on the wheels just before touchdown to produce rotation.
Without going into critical detail, this inventor would venture the comment vis-a-vis Prior Art, that many of the devices show an apparent lack of consideration of the magnitude of the actual forces, masses and speeds involved in inducing adequate prerotation of the massive wheels and tires used on the landing gear of today's aircraft.
In most instances, the desirability of prerotation was reiterated in a general way, but numerous specifics of the actual problem and task were not dealt with. It is the judgment of this inventor that any mechanical connection between the aircraft wheel and any prerotation device is highly vulnerable to damage or destruction and consequent damage to the aircraft tires if the induced prerotation RPM deviates sufficiently from the forced RPM which will begin immediately upon touchdown; yet no patented device studied by this inventor appears to provide actual means to achieve, confirm, and sustain true synchronous rotation.
Inasmuch as the exact instant of landing-wheel touchdown contact cannot be precisely known in advance, and inasmuch as the ground speed of the aircraft is decreasing continuously during final approach, it will be seen that the achievement of exact synchronism between tires and runway can only be realized by defining a "window of time" which begins early enough to contain, with certainty, the moment of touchdown; and from the beginning of which an actual measured synchronism will be achieved and maintained, accurately and continuously tracking ground speed and adjusting RPM to conform to it. Depending upon aircraft size and landing speeds, number of and weights of wheels, load and other variables, such a window of time may typically be regarded as beginning sixty to ninety seconds after lowering of landing wheels, and in virtually all cases thirty to sixty seconds before the expected moment of touchdown, and terminating only after all landing wheels are on the ground rolling, including the all-important nose wheel(s).
The smooth jolt-free landings which sychronous prerotation of aircraft landing wheels can provide is long overdue. Regardless of how heavy, massive and well-braced the landing gear assemblies of large aircraft may be, the severe jolt and shock of these landing gear holding their 500-lb. tires as they ease down to the concrete runway hanging static, at zero RPM, and are revved up by sheer brute force to a sudden 1,100 RPM in the space of half a second, is a jolt which almost certainly transmits damaging recoil forces throughout the entire frame of the aircraft. Fatigue of metals is almost sure to result, because of the magnitude of the forces involved and the extreme shortness of time.
Under these conditions, the wheel balancing so familiar to motorists, and so important to tire life are difficult to maintain beyond the first landing, for the smoky cloud of rubber trailing out beyond the rudders of our large aircraft during this critical half-second are burned-off parts of the $1,000 tires. As a result of this destructive abrasion, aircraft tires today have a useful life of about 125 landings before being replaced or recapped. These 125 landings figure out to less than 1,000 ground miles per tire, of which only about half are torque (braking) miles and the rest are rolling miles. This compares poorly with the typical life of truck tires of roughly the same size which commonly travel 25,000 to 30,000 miles before replacement. For safety as well as economy and longer tire life, prerotation is long overdue.
The very substantial safety benefit and major savings in tire costs arising out of use of the prerotation system would seem to make it a desirable alternative to the destructive abuse of several $1,000 400-lb. aircraft tires.
While the Prerotation Control System as outlined is envisioned as being developed and produced to be retrofitted on a wide range of existing aircraft with little or no minor modification of them, it is also a system which could be made available to aircraft manufacturers for incorporation into new and future aircraft.